In FSK modulation, bits or sequences of bits are represented by signals of at least two different frequencies. Due to technical constraints, the modulation properties are not always stable in time. For example, modulation properties may be affected by temperature variations, power supply variations, component tolerances, and relative motion between the transmitter and receiver. Particularly in the case of, for example, a vehicle security system, the transmitter is typically mounted in a small key fob or key handle, and is a low-cost (poor tolerance) miniature circuit, supplied by a miniature battery. With such a transmitter, it is difficult to control precisely the frequencies that will be generated by the transmitter. The frequencies may also wander during a single transmission. Also, the times at which the FSK signals will be transmitted may be unknown at the receiver.
Non-coherent techniques are known for demodulating such unpredictable FSK signals. However, as well as having a high computational overhead, conventional non-coherent techniques are highly vulnerable to the effects of interference from other signals in the same frequency range as the FSK signals of interest. Non-coherent techniques may be particularly affected by coherent interference in the form of one or more relatively stable interfering frequencies. Such coherent interference is often generated by electric lighting or by electronic equipment, for example, digital circuits having a stable clock frequency. In a vehicle environment, there are many independent circuits that may provide close-frequency interference.
Techniques are known for suppressing the effects of noise and interference. However, the more effective of these techniques rely on a reference source of the noise or interference to be suppressed. For example, the reference source may be a second receiver at a different location from a first receiver to provide a spatially different received signal. However, such a second receiver adds significant cost, and is often highly inconvenient to implement. The reference source may alternatively be in the form of a pre-programmed signal generator or mathematical model. However, the use of a pre-programmed reference assumes advance knowledge of the interference. Also, a pre-programmable reference is not adaptable to changing conditions nor even to a range of different conditions.
In summary, there remain many significant problems in providing a technique which is able to operate with conventional poor tolerance transmitters, and which can provide efficient, low cost and yet robust, processing and demodulation of FSK signals, and also provide relatively high immunity to close-frequency interference.